Access state, configuration, and plan data

There are various points at which the provider needs access to the data from the practitioner's configuration, Terraform's state, or generated plan. The same patterns are used for accessing this data, regardless of its source.

The data is usually stored in a request object:

func (r ThingResource) Create(ctx context.Context,
    req resource.CreateRequest, resp *resource.CreateResponse)

In this example, req holds the configuration and plan, and there is no state value because the resource does not yet exist in state.

Get the Entire Configuration, Plan, or State

One way to interact with configuration, plan, and state values is to convert the entire configuration, plan, or state into a Go type, then treat them as regular Go values. This has the benefit of letting the compiler check all your code that accesses values, but requires defining a type to contain the values.

Use the Get method to retrieve the first level of configuration, plan, and state data.

type ThingResourceModel struct {
    Address    types.Object `tfsdk:"address"`
    Age        types.Int64  `tfsdk:"age"`
    Name       types.String `tfsdk:"name"`
    Pets       types.List   `tfsdk:"pets"`
    Registered types.Bool   `tfsdk:"registered"`
    Tags       types.Map    `tfsdk:"tags"`
}

func (r ThingResource) Create(ctx context.Context,
    req resource.CreateRequest, resp *resource.CreateResponse) {
    var plan ThingResourceModel

    diags := req.Plan.Get(ctx, &plan)

    resp.Diagnostics.Append(diags...)

    if resp.Diagnostics.HasError() {
        return
    }

    // values can now be accessed like plan.Name.ValueString()
    // check if things are null with plan.Name.IsNull()
    // check if things are unknown with plan.Name.IsUnknown()
}

The configuration, plan, and state data is represented as an object, and accessed like an object. Refer to the object type documentation for an explanation on how objects can be converted into Go types.

To descend into deeper nested data structures, the types.List, types.Map, and types.Set types each have an ElementsAs() method. The types.Object type has an As() method.

Get a Single Attribute or Block Value

Use the GetAttribute method to retrieve a top level attribute or block value from the configuration, plan, and state.

func (r ThingResource) Read(ctx context.Context,
    req resource.ReadRequest, resp *resource.ReadResponse) {
    var name types.String

    diags := req.State.GetAttribute(ctx, path.Root("name"), &name)

    resp.Diagnostics.Append(diags...)

    if resp.Diagnostics.HasError() {
        return
    }

    // ...
}

When Can a Value Be Unknown or Null?

A lot of conversion rules say an error will be returned if a value is unknown or null. It is safe to assume:

• Required attributes will never be null or unknown in Create, Read, Update, or Delete methods.
• Optional attributes that are not computed will never be unknown in Create, Read, Update, or Delete methods.
• Computed attributes, whether optional or not, will never be null in the plan for Create, Read, Update, or Delete methods.
• Computed attributes that are read-only (Optional is not true) will always be unknown in the plan for Create, Read, Update, or Delete methods. They will always be null in the configuration for Create, Read, Update, and Delete methods.
• Required attributes will never be null in a provider's Configure method. They may be unknown.
• The state never contains unknown values.
• The configuration for Create, Read, Update, and Delete methods never contains unknown values.

In any other circumstances, the provider is responsible for handling the possibility that an unknown or null value may be presented to it.